The present invention relates to a process for manufacturing clad pipe which consists of two layers, i.e., an inner pipe and an outer pipe, made of dissimilar metallic materials and metallurgically bonded to each other, the clad pipe being suitable for use in oil and gas wells, pipelines, and chemical plants.
Clad pipe can be designed such that the inner and outer pipes of which it is formed perform different functions and enable the clad pipe as a whole to exhibit the desired properties. For example, in clad pipe having an inner pipe of an Ni-base alloy and an outer pipe of steel, corrosion resistance is provided by the inner pipe while strength is mainly guaranteed by the outer pipe. Such clad pipe is used in oil and gas wells and chemical plants, where corrosion resistance of the interior surface is of great importance.
Clad pipe is generally manufactured by inserting an inner pipe into an outer pipe and subjecting the telescoped two pipes together to hot working such as hot rolling or hot extrusion. During the hot working, the two pipes are reduced in diameter to urge the outer pipe in contact with the inner pipe, and thereafter they are metallurgically bonded by thermal diffusion, i.e., by diffusion bonding under action of the heat applied by hot working.
In order to ensure that the two pipes are metallurgically bonded with good adhesion by diffusion bonding, it is necessary to pretreat the two pipes so as to make the surfaces to be bonded clean and smooth and protect them from oxidation. For this purpose, after the outside surface of the inner pipe and the inside surface of the outer pipe are polished and washed thoroughly, the two pipes are telescoped together (by inserting the inner pipe into the outer pipe) and the space between the two pipes is sealed by seal welding at each end such that the space can be freed of oxygen by evacuating the space. This process is complicated and greatly increases the manufacturing costs of clad pipe.
As a less expensive process for manufacturing clad pipe in which the above-described evacuation step is eliminated, it is known to interpose a low-melting insert between the inner and outer pipes so as to facilitate diffusion bonding.
Japanese Patent Application Laid-Open No. 59-159284 (1984) describes a process in which an inner pipe made of a metallic material having a relatively high coefficient of thermal expansion is fitted into an outer pipe with an Ni foil or an Ni-P plated layer interposed between the two pipes as an insert. The assembled two pipes are heated to join them by the thermal stress caused by expansion of the inner pipe.
In the process described in Japanese Patent Applications Laid-Open Nos. 62-78783 (1987) and 62-72423 (1987), a tape of Ni foil having a phosphorus coating plated on each surface thereof is spirally wound around an inner pipe to form an insert. The inner pipe is then inserted into an outer pipe, and the assembled two pipes are bonded either by subjecting them to cold working for reduction in diameter followed by heat treatment for thermal diffusion, or by subjecting them to hot working.
Japanese Patent Application Laid-Open No. 1-197081 (1989) describes a process in which a layer of a low-melting Ni-base alloy containing Si and B is formed by thermal spraying and is used as an insert.
The above-described known processes using an insert have the following disadvantages.
(1) In the process where diffusion bonding of assembled inner and outer pipes is performed by hot working, the hot working causes a corrosion-resistant alloy such as an Ni-base alloy, which is often used to form the inner pipe, to crack. In addition, the difference in deformation resistance between the inner and outer pipes made of different materials interferes with smooth hot working.
(2) In the process where a tape of a metallic foil is spirally wound around an inner pipe to form an insert, air remains between overlapped layers of the wound tape to form many air pockets, which result in the formation of voids in the bonded area between the two pipes even when they are bonded by cold working followed by heat treatment for thermal diffusion. In order to remove these voids, it is necessary to perform cold drawing, for example, as an additional step, which adds to the manufacturing costs.
(3) In the process where an Ni-P plated layer is used as an insert, an Ni-P eutectic is formed in the bonded area, thereby decreasing the bonding strength and deteriorating the reliability of the process.
(4) The process where an insert is formed by thermal spraying involves the same problem as described in (2) above, since the sprayed layer contains many voids.